1 - print the parabolic templates below at exactly 72 dots per inch.  the diameter of the large circle should be 2.43 inches or 61.7 mm (1/2 wavelength).

2 - use the templates to cut out 1 to 3 flat forms made of (non-metallic) cardboard, foamcore, styrofoam, balsa, or plywood.

3 - punch or drill center holes to fit snugly over existing wifi antenna (one omni-directional monopole).

4 - cut an aluminum sheet from the side of an empty soda pop or beer can, about 3.6 or 5.2 inches tall/long, about 8.2 inches wide/around/arc.  steel, tin, etc also work fine.

5 - wrap aluminum sheet around one curved side of form(s) to make a parabolic reflector, and attach it to the forms with (non-metallic) rubber bands, tape, silicone, or glue.

6 - optionally, attach a short grounding wire (18 to 22 awg) from the aluminum sheet to wifi antenna coax shield, or chassis / power-supply ground.

construction notes - reflector width is not critical, wider gives more gain.  i haven't thoroughly investigated reflector heights (please email your results!).  tall 24oz aluminum soda cans have given best performance so far, and without grounding.  sections of aluminum window screen or galvanized steel mesh have been successful for lower outdoor wind-load.

      speed of light in vacuum  =  299792458 meters/sec
refractive index of air at stp  =          1.0002926
  speed of light in air at stp  =  299704764.38594 meters/sec

channel      freq       cyc/sec     wl in vac   wl in air     wavelength/4 in air    % of wifi 6
wifi 1    2.412 ghz = 2412000000   124.292 mm  124.256 mm   31.064 mm ~= 1.223 inch   101.036 %
wifi 6    2.437 ghz = 2437000000   123.017 mm  122.981 mm   30.745 mm ~= 1.210 inch   100.000 %
wifi 11   2.462 ghz = 2462000000   121.768 mm  121.732 mm   30.433 mm ~= 1.198 inch    98.985 %

one pixel on the template below  =  1/72 inch  ~= .014 in  ~= .353 mm,
one wifi channel delta bandwidth  = .005 ghz  =  50 mhz  ~=  wl/4 0.0025 inch  ~= .2 % of wifi 6,
therefore changing wl/4 +or- 1 pixel (1/72 inch) covers the range from wifi channel 1 to 11.

also, due to a slight original miscalculation, this template is correct for wifi channel 4 (not 6)...
the error in wl/4 is ~.005 inches, which is ~.36 pixels or about 1/3 of a pixel smaller for wifi 6. 
(you could scale your printer output to 99.59% instead of 100% - updated template in progress... :)